Nonaqueous-electrolyte batteries, in particular, lithium secondary batteries, are recently attracting attention as power sources for portable appliances such as portable telephones, PHSs (simplified portable telephones), small computers, etc., power sources for power storage, and power sources for electric motorcars. In general, a lithium secondary battery is constituted of a positive electrode comprising a positive active material as a main component, a negative electrode comprising a negative-electrode material as a main component, and a nonaqueous electrolyte, and is produced by covering a power-generating element comprising the positive and negative electrodes with a sheath. A lithium-containing transition metal oxide and a carbonaceous material are mainly used respectively as the positive active material and negative-electrode material contained in the lithium secondary battery. Of such materials, a graphite is a material suitable for use in batteries having a high energy density because it has flat-potential characteristics. Widely known nonaqueous electrolytes are ones comprising an organic solvent comprising ethylene carbonate as a main component and an electrolyte, e.g., lithium hexafluorophosphate (LiPF6), dissolved in the solvent.
Since the use of ethylene carbonate is apt to cause electrolyte solidification at low temperatures because it has a high melting point, a technique for improving various properties including low-temperature properties is known which comprises using an organic solvent having a high permittivity and a lower melting point (e.g., propylene carbonate). However, especially in the case of using a graphite in the negative electrode, there has been a problem that the organic solvent such as propylene carbonate decomposes on the graphite negative electrode and, hence, charge/discharge cannot be conducted at a high efficiency.
A technique for overcoming that problem has been disclosed which comprises adding vinylene carbonate or the like to a nonaqueous electrolyte to thereby inhibit the organic-solvent decomposition described above (see, for example, patent document 1). Specifically, there is a statement therein to the effect that the vinylene carbonate or the like is selectively decomposed on the graphite negative electrode in the charge conducted first after battery fabrication (this charge is hereinafter referred to as “first charge”), whereby a protective coating film permeable to lithium ions is formed on the surface of the graphite negative electrode and the decomposition of the organic solvent such as propylene carbonate is inhibited. However, this technique was insufficient in the effect of inhibiting the decomposition of the organic solvent during first charge. Furthermore, there has been a problem that vinylene carbonate has poor oxidation resistance and decomposes on the positive-electrode side and, hence, the addition of vinylene carbonate in a large amount reduces battery performances.
On the other hand, quaternary ammonium salts have been frequently used for a long time as an electrolyte material for electric double-layer capacitors. However, with respect to application to batteries, the only technique which has been reported is to use a quaternary ammonium salt in a nonaqueous-electrolyte battery which employs a conductive polymer (polyacene) as an electrode material and in which lithium ions do not participate in the electrode reactions (see patent documents 2 and 3). No advantage has been found in the use thereof in electrolytes for nonaqueous-electrolyte batteries in which lithium ions participate in electrode reactions. On the other hand, some of imidazolium salts and the like which are a kind of quaternary ammonium salt have a property of room temperature molten salts, i.e., being liquid at room temperature. It has hence been proposed to use such a salt as a main component of an electrolyte which eliminates the necessity of using any organic solvent such as those for use in general nonaqueous-electrolyte batteries. (See patent document 4).
[Patent Document 1] JP-A-11-67266
[Patent Document 2] JP-A-62-31958
[Patent Document 3] JP-A-2-177271
[Patent Document 4] JP-A-2002-110230
The invention has been achieved in view of the problems described above. An object of the invention is to provide a nonaqueous-electrolyte battery having a high charge/discharge efficiency and excellent high-rate discharge characteristics.